To produce oil and gas from hydrocarbon reservoir, a borehole of tapered and often times deviated geometry is first drilled through geological formations. The hydrocarbon-bearing formation then is drilled with a specially designed reservoir drilling fluid, which may comprise various additives, such as starches and calcium carbonate, that are soluble or breakable by acid, oxidizers, or enzymes, or a combination of these chemicals.
Once the desired borehole in the hydrocarbon reservoir is drilled, production tubes and/or screens are run to the bottom of the borehole and placed against the desired formations for hydrocarbon production. Often times, especially when the hydrocarbon-bearing formations consist of poorly cemented sands, some kind of sand control methods or devices are used to prevent sand particles from the formation from entering and plugging up the production screens and tubes in order to extend the life of the well.
One of the typical sand control methods is to fill the annular space between the wellbore and the production screens with specially sized sand, which is usually larger than the formation sand and commonly known as gravel pack sand. The process to place the sized sand behind the production screen is known as a gravel pack operation.
In order to be able to fill the annular space with sand completely and successfully, the hydrocarbon-bearing formation should have been previously covered with a thin layer of firm and impermeable filter cake formed by the reservoir drilling fluid. This thin and impermeable filter cake may prevent the gravel pack fluid from entering the formation, which when occurring at an uncontrollable rate, would result in gravel pack failure.
After the gravel pack sand has been successfully placed, the filter cake existing between the gravel pack sand and the formation needs to be removed before the flow of hydrocarbon is initiated. Without the removal of the filter cake, plugging of the production screen by the filter cake could occur and would result in a production impairment.
To destroy the filter cake that is now behind the gravel pack sand, various chemicals, breakers and mechanical devices have been developed and used. For example, hydrochloric acid is often delivered by a separate operation to soak the gavel pack sand and filter cake with the aid of wash cups. The mechanical wash cups attached to the end of a work string must be picked up at the surface and lowered to the bottom through the inside of the screen. The hydrochloric acid is then pumped through the gravel pack sand repeatedly. The goal of this exercise is to destroy a large amount of the acid-soluble and acid-breakable components in the filter cake.
Other breakers, such as oxidizers and enzymes, may also be delivered to destroy oxidizer- and enzyme-breakable organic components, such as starch polymers. However, these breakers are considered less efficient in several ways. First, they are not effective in destroying acid-soluble and acid-breakable inorganic components in the filter cake, such as calcium carbonate. As a result, acid-soluble and breakable components will remain behind the gravel pack sand and may subsequently cause impairment during the production of the well. Second, many oxidizing breakers have compatibility issues with certain brines. They may react with the brine and create undesirable by-products, such as Cl2 and Br2 gases. This reaction will occur even before the breakers were pumped down to attack the filter cake. Third, in addition to brine compatibility issues, enzyme breakers also have a temperature issue. Most enzyme breakers will lose reactivity in highly concentrated divalent brines, and at temperatures above 200° F.
The above breakers are normally pumped on a separate trip after the gravel pack sand has been set. They are not pumped during the gravel pack operation because they can create precarious conditions for the operation. For instance, the acid-based breakers can destroy the filter cake during gravel pack operation, and consequently result in high fluid loss and premature failure in the gravel pack operation.
Pumping oxidizers and enzyme breakers with gravel pack sand may cause inconsistent application of oxidizers and enzyme breakers to the filter cakes. Since most of the solid oxidizers and enzyme breakers are organic materials with relatively low specific gravity and small particle size, they tend to be pushed toward the screen rather than toward the filter cake where the reaction needs to take place. As a result, the concentration and distribution of these breakers in the gravel pack sand is likely to be erratic, making the filter cake removal less effective.
Microencapsulation is one technique used to deliver wellbore chemicals downhole. The microencapsulation process and application of microencapsulated oil field chemicals, such as scale inhibitors, corrosion inhibitors, surfactants, bactericides, paraffin dispersants, pourpoint modifiers, cement additives, fracture fluid cross linkers, emulsion breaking chemicals, chemical tracers, radioactive tracers, and asphaltene treatment chemicals, using condensation product of hydroxyacetic were disclosed in U.S. Pat. No. 4,986,354. The encapsulated special chemicals are injected along with water into oil wells. Disintegration of the encapsulating polyglycolic polymer in the presence of water allows the encapsulated chemicals to be released to achieve desired reactions.
Microencapsulation of pesticides, insect growth regulators, and other organic compounds in biodegradable polymers from the group consisting of polylactic acid and copolymers of lactic and glycolic acids was disclosed in U.S. Pat. No. 4,272,398.
None of the methods above efficiently deliver the necessary breakers to a filter cake. Thus, there exists an on-going need and desire for breakers which provide a slow release mechanism to initiate the disintegration of filter cakes so that gravel pack operations can be continued.